QtIFW

Fixed reading stderr output of sudo

After the sudo process finished, we need to make sure to read any
buffered bytes from the stderr pipe. Without this, the stderr data which
was shown in the message box often ended up being incomplete. For
instance, three failures to enter the correct root password would show a
message box saying

  Sorry, try again.
  Sorry, try again.

Instead of

  Sorry, try again.
  Sorry, try again.
  Sorry, try again.
  sudo: 3 incorrect password attempts

Change-Id: Ib1e7aa40d95f9bf140ba1978fdb9be92cad926b6
Reviewed-by: Karsten Heimrich <karsten.heimrich@theqtcompany.com>
Reviewed-by: Katja Marttila <katja.marttila@theqtcompany.com>